A Review of Criticality Accidents A Review of Criticality Accidents

operator stood to pour the solution. The second
operator stood on the floor and held the funnel.
Completion of the four batches concluded the threeperson
team’s work for that day.
The next day, 30 September, the three operators
began dissolving the final three batches that would be
required to complete the job. After transferring batches
five and six, the pouring of the seventh batch was
begun around 10:35. Almost at the end of the pour
(183 g of uranium were recovered from the flask) the
gamma alarms sounded in this building and in the two
nearby commercial fuel buildings. Workers evacuated
from all buildings according to prescribed plans and
proceeded to the muster area on site. At this location,
gamma ray dose rates far above background were
detected and it was suspected that a criticality accident
had occurred and was ongoing.
The muster location was then moved to a more
remote part of the plant site where dose rates were near
to background values. The excursion continued for
nearly twenty hours before it was terminated by
deliberate actions authorized and directed by govern-
54
A
Authorized Procedure
2.4 kg U O 3 8
Batch
B
Executed Procedure
2.4 kg U 3 O 8
Batch
Dissolver
10 Stainless Steel
Bucket
Nitric Acid
Nitric Acid
5 Flask
Geometrically
Favorable Vessel
Figure 34. Authorized and executed procedures.
4 Bottles
Product Solution UO 2 (NO 3 ) 2
Precipitation
Tank B
Product
Solution
UO 2 (NO 3 ) 2
ment officials. During this time there were several
noteworthy aspects of this accident. First, the JCO
Company was not prepared to respond to a criticality
accident - the gamma alarms were not part of a
criticality accident alarm system. In fact, the license
agreement stated that a criticality accident was not a
credible event. Thus expertise and neutron detectors
had to be brought in from nearby nuclear facilities.
Various monitoring devices at the facility as well as the
nearby Japan Atomic Energy Research Institute
(JAERI), recorded the excursion history. These
showed, after a large initial spike, that the power level
quasi-stabilized, dropping gradually by about a factor
of two over the first ~17 hours.
About 4.5 hours after the start of the accident,
radiation readings taken at the site boundary nearest to
a residential house and a commercial establishment
showed combined neutron and gamma ray dose rates
of about 5 mSv/hour. At this time the Mayor of Tokaimura
recommended that residents living within a
350 m radius of the JCO plant evacuate to more remote
locations. After 12 hours, local, Ibaraki-ken, govern-